Printed circuit board manufacture

ABSTRACT

A process used during manufacture of printed circuit boards comprises protecting metal pads and/or through-holes to provide a tarnish-resistant and solderable coating. In the method, the pads and/or through-holes are bright-etched, metal plated, preferably by an immersion process, and treated with a tarnish inhibitor. The tarnish inhibitor may be incorporated into the immersion plating bath. The metal plating is usually with silver or bismuth and the pads and/or through-holes comprise copper.

FIELD OF THE INVENTION

[0001] In the production of a printed circuit board (PCB), in a first(multi-step) stage a “bare board” is prepared and in a second(multi-step) stage, various components are mounted on the board. Thepresent invention relates to the final steps in the manufacture of thebare board, in which the bare board is coated with a protective layerprior to passing to the second production stage.

PRIOR ART

[0002] There are currently two types of components for attachment to thebare boards in the second stage referred to above: legged componentse.g. resistors, transistors, etc., and, more recently, surface mountdevices. Legged components are attached to the board by passing each ofthe legs through a hole in the board and subsequently ensuring that thehole around the leg is filled with solder. Surface mount devices areattached to the surface of the board by soldering with a flat contactarea or by adhesion using an adhesive.

[0003] In the first stage referred to above, a board comprising aninsulating layer, a conducting circuit pattern and conductive padsand/or through-holes is produced. The board may be a multi-layer boardhaving more than one conducting circuit pattern positioned betweeninsulating layers or may comprise one insulating layer and oneconducting circuit pattern.

[0004] The through-holes may be plated through so that they areelectrically conducting and the pads which form the areas to which thesurface mount components will be attached in the subsequentcomponent-attachment stage, are also electrically conducting.

[0005] The conductive areas of the circuit pattern, pads andthrough-holes may be formed from any conductive material or mixtures ofdifferent conductive materials. They are generally however, formed fromcopper. Since over time copper tends to oxidise to form a copper oxidelayer with poor solderability, prior to passing to the second,component-attachment stage, a protective layer is coated over the padsand/or through-hole areas where it is desired to retain solderability toprevent formation of a poorly solderable surface layer of copper oxide.

[0006] While there is more than one way for preparing the bare boards,one of the most widely used processes for making the bare boards isknown as the “solder mask over bare copper” (SMOBC) technique. Such aboard generally comprises an epoxy-bonded fiberglass layer clad on oneor both sides with conductive material. Generally, the board will be amulti-layer board having alternate conductive layers which comprisecircuit pattern, and insulating layers. The conductive material isgenerally metal foil and most usually copper foil. In the SMOBCtechnique, such a board is obtained and holes are drilled into the boardmaterial using a template or automated drilling machine. The holes arethen “plated through” using an electroless copper plating process whichdeposits a copper layer on the entirety of the board: both on the upperfoil surfaces and on the through-hole surfaces.

[0007] The board material is then coated with a light sensitive film(photo-resist), exposed to light in preselected areas and chemicallydeveloped to remove the unexposed areas revealing the conductive areaswhich are the plated through-holes and pads. Generally, in the nextstep, the thickness of the metal foil in the exposed areas is built upby a further copper electroplating step. A protective layer of an etchresist, which is usually a tin lead alloy electroplate composition isapplied over the exposed and thickened copper areas.

[0008] The photo-resist is then removed exposing the copper for removaland the exposed copper surface is etched away using a copper etchingcomposition to leave the copper in the circuit pattern finally required.In the next step, the tin-lead alloy resist is stripped away.

[0009] Since components will not be attached to the copper circuittraces, it is generally only necessary to coat the solder for attachingthe components over the through-hole and pad areas but not the traces.Solder mask is therefore applied to the board to protect the areas wherethe solder coating is not required, for example using a screen printingprocess or photo-imaging technique followed by development and,optionally curing. The exposed copper at the holes and pads is thencleaned and prepared for solder coating and the protective soldercoating subsequently applied, for example by immersion in a solder bath,followed by hot air leveling (HAL) to form a protective solder coatingon the areas of copper not coated with solder mask. The solder does notwet the solder mask so that no coating is formed on top of the soldermask protected areas.

[0010] At this stage, the board comprises at least one insulating layerand at least one conductive layer. The conductive layer or layerscomprise a circuit trace. The board also comprises a pad or pads and/orthrough-hole(s) protected from tarnishing by a layer of solder. A singleconductive layer may comprise either a circuit trace or pad(s), or both.Any pads will be part of a conductive layer which is an outerlayer of amulti-layer board. The circuit traces on the board are coated withsolder mask.

[0011] Such a board is ready to proceed to the second stage forattachment of the components. In this second stage, generally attachmentof the components is achieved using solder: firstly a layer of solderpaste (comprising solder and flux) is applied onto the boards, generallyby printing and the components are positioned on the printed boards. Theboard is then heated in an oven to produce fusion of the solder in thesolder paste, which forms a contact between the components and theboard. This process is known as reflow soldering. Alternatively a wavesoldering process is used in which the board is passed over a bath ofmolten solder. In either case additional solder is used over and aboveany protective solder coating.

[0012] The additional complications of attaching both legged componentsand the surface mount devices and the special requirements for mountingmany small closely spaced components have resulted in increased demandson the surface protection coating for the conductive metal to which thecomponents will be attached on the PCB's. It is essential that thefinish applied by the bare board manufacturer does not leave a pad withan uneven surface as this increases the risk of electrical failure. Itis also essential that the protective coating does not interfere withthe subsequent solder step, thereby preventing formation of a good,conducting bond between the bare board and components. An extra step inwhich the protective coating is removed would be undesirable.

[0013] As explained above, the conductive metal surfaces are generallyformed of copper and the protective surface must be applied at the endof the first stage to prevent the formation of non-solderable copperoxide on the copper surfaces prior to the component attachment. This isparticularly important because, generally speaking, the first stage andthe second, component-attachment stage will be carried out at completelydifferent sites. There may therefore be a considerable time delaybetween formation of conducting pads and/or through-holes and thecomponent-attachment stage, during which time oxidation may occur.Therefore, a protective coating is required which will retain thesolderability of conducting material and enable a soldered joint to bemade when the components are attached to the bare boards.

[0014] The most common protection coating presently used is tin/leadsolder, generally applied using the “HAL” (hot air leveling) process, anexample of which is described in detail above.

[0015] HAL processes are limited because it is difficult to apply thesolder evenly and the thickness distribution produced by the use of HALprocesses makes it difficult to reliably attach the very small andclosely spaced components now being used.

[0016] Several replacement treatments for the HAL coating of a solderlayer are being introduced. The coatings must enable formation of areliable electrical contact with the component. They should also be ableto stand up to multiple soldering steps. For example, as describedabove, there are now both legged and surface mount components forattachment to the bare boards and these will generally be attached in atleast two soldering operations. Therefore, the protective coatings mustalso be able to withstand at least two soldering operations, so that theareas to be soldered in a second operation remain protected during thefirst operation.

[0017] Alternatives to the tin/lead alloy solder used in the HALprocess, which have been proposed include organic protection, immersiontin or tin/lead plating and nickel/gold plating. In the nickel/goldprocess electroless plating of the copper surfaces is carried out inwhich a primer layer of nickel is applied onto the copper followed by alayer of gold. This process is inconvenient because there are manyprocess steps and in addition, the use of gold results in an expensiveprocess.

[0018] Organic protection for the copper pads during storage andassembly prior to soldering have also been effected using flux lacquer.Its use is generally confined to single-sided boards (i.e. boards whichhave conductive pads on only one side). The coating is generally appliedby dip, spray or roller coating. Unfortunately, it is difficult toprovide a consistent coating to the board surfaces so limited lifeexpectancy, due to the porosity of the coating and to its inconsistentcoating thickness, results Flux lacquers have also been found to have arelatively short shelf life A further problem is that in thecomponent-attachment stage, if reflow soldering is to be used to attachthe components, the components are held in place on the underside of theboards with adhesive. In cases where the flux lacquer is thick, theadhesive does not bond the component directly to the printed board, butinstead forms a bond between the adhesive and the lacquer coating. Thestrength of this bond can drop during the fluxing and soldering stepcausing components co be lost during contact with the solder baths.

[0019] One other alternative currently being used ispassivation/protection treatment based upon the use of imidazoles ortriazoles in which copper-complex compounds are formed on the coppersurface. Thus, these coatings chemically bond to the surface and preventthe reaction between copper and oxygen. However this process isdisadvantageous because it tends to be inadequate for withstandingsuccessive soldering steps so that the high temperatures reached in afirst soldering step tend to destroy the layer which cannot withstand asubsequent soldering operation needed to mount further components. Oneexample of such a process is given in EP-A-0428383, where a process isdescribed for the surface treatment of copper or copper alloyscomprising immersing the surface of copper or copper alloy in an aqueoussolution containing a benzimidazole compound having an alkyl group of atleast 3 carbon atoms at the 2-position, and an organic acid.

[0020] Processes are also known which provide coatings usingcompositions which comprise silver.

[0021] The three common complexing systems for electroless silverplating processes are either ammonia-based, thiosulphate-based orcyanide-based.

[0022] The ammonia systems are disadvantageous because theammonia-containing silver solutions are unstable and explosive azidesmay tend to form. Thiosulphate systems are disadvantageous for use inthe electronics industry because sulphur compounds in the silvercoatings formed result in poor solderability so that in the subsequentcomponent-attachment step, a poor electrical contact may be formedbetween the bare board and the component.

[0023] The cyanide-based systems are disadvantageous due to the toxicityof the plating solutions.

[0024] In U.S. Pat. No. 5,318,621 an electroless plating solutioncontaining amino acids as rate enhancers for depositing silver or goldonto a nickel coating overlying copper on a circuit board is disclosed.It is described that neither gold nor silver electroless plating bathsbased on thiosulphate/sulphate will plate directly onto copper becausethe copper rapidly dissolves without allowing a silver or gold coatingto form. In the introduction of the reference, “Metal FinishingGuidebook & Directory” (1993 edition), silver plating solutionscomprising silver nitrate, ammonia and a reducing agent such asformaldehyde are mentioned.

[0025] U.S. Pat. No. 4,863,766 also discloses electroless silverplating, using a cyanide-based plating solution. In Metal Finishing(1983) 81(i), pp 27-30 Russev described immersion silvering of copperpowder from a plating solution containing silver nitrate and a nitrogencomplexing agent In Metal Finishing (1960) August, p 53 Geld described asilver coating process involving an initial bright dip of the brass orcopper substrate, followed by a silver plating step in which a thickcoating of silver is plated from a solution of silver nitrate andpotassium iodide. The process is for plating of electrical contacts toincrease conductivity.

[0026] In JP-A-04-110474 a base material is plated with silver, driedand subsequently treated with a mercaptan compound to prevent tarnish.

[0027] In DE-C-4316679 base metals such as copper are coated withpalladium in a two-step process including a first step in which thesurface is contacted with a bath containing a palladium salt and anoxidizing agent, and in the second step with a bath containing apalladium salt, a complexing agent and formic acid or formic acidderivative. The latter bath may also contain stabilizers for the bathitself, which stabilize the bath against decomposition or “plating-out”.It is suggested that the copper substrate should previously be etchedusing a non-bright etch bath including persulphate. However, suchpretreatment steps tend to produce relatively porous coatings. Theinventors there minimize the porosity of the coating by using thetwo-step process in the first of which a very thin coating is formed.This reference warns against using silver as corrosion protection due tomigration.

[0028] The present invention relates to a displacement immersion metalplating in which a more electropositive metal displaces a lesselectropositive metal at the surface to be coated. Ions of the moreelectropositive metal oxidize the substrate metal. A displacementplating process differs from an electroless process because the silvercoating forms on the surface of a metal by a simple displacementreaction due to the relative electrode potentials of the oxidisablemetal of the surface to be protected and of the silver ionsrespectively.

[0029] It is reported in for example “Modern Electroplating” by F. A.Lowenheim, published by J. Wiley & Sons (1963) that silver will plate bydisplacement on most base metals but that immersion plated silver ispoorly adherent. F. A. Lowenheim there suggests that when electroplatingbase metals with silver, it is necessary to deposit first a thin film ofsilver on the work piece from a high-cyanide strike bath to ensure goodadhesion of the subsequent electroplated silver layer.

SUMMARY OF THE INVENTION

[0030] The present invention aims to provide an alternative to thesolder protection coating for the copper or other conducting surfaces ofbare boards which require protection from tarnishing between bare boardmanufacture and the component-attachment stage.

[0031] In accordance with the present invention, there is provided amethod for coating a PCB comprising an insulating layer and a conductinglayer, with metal pads and/or through-holes in which the pads and/orthrough-holes are provided with an anti-tarnish coating, the processcomprising contacting the pads and/or through-holes with a bright-etchcomposition in a bright-etch step; and subsequently immersion platingthe etched pads and/or through-holes in a metal-plating step to formsolderable plated metal surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a graphical representation illustrating an exampledescribed in the present application.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The insulating layer and conducting layer of the PCB will be asdescribed immediately above. They may comprise the insulating layer andconducting circuit pattern of any conventional PCB, respectively. Thepads and/or through-holes for plating are those areas of the PCB forwhich solderability must be maintained for attachment of components inthe subsequent soldering steps for component attachment.

[0034] The bright-etch step comprises contacting the pads and/orthrough-holes with a bright-etch composition. Such compositions arealready known in the industry for other applications and they produce abright smooth cleaned surface on the conducting metal from which thepads and/or through-holes are formed. In contrast, non-bright etchcompositions, such as those which are based on persulphate providemicroroughened, cleaned surfaces. The use of the bright-etch step allowsthe formation of a dense, nonporous metal coating, which is particularlysuitable for a subsequent soldering step.

[0035] Suitable bright-etch compositions are generally aqueous and maybe based for example on one or mixtures of more than one of hydrogenperoxide, sulphuric acid, nitric acid, phosphoric acid or hydrochloricacid. The bright-etch compositions generally include at least onecomponent which will tend to modify the dissolution of copper inbright-etch compositions.

[0036] Particularly preferred bright-etch compositions where the metalsurface of the pads and/or through-holes comprises copper or a copperalloy are, for example, as described in JP 62-188785 A2 (comprising5.1-10.2 moles/l nitric acid, 4.6-9.2 moles/l sulphuric acid, 0.01moles/4 zinc nitrate and 0.4 moles/l copper nitrate in aqueoussolution); JP 60-190582 (comprising for example 20-50% by weightsulphuric acid (96%), 10-25% by weight nitric acid (67.5%), 0.5-1% byweight hydrochloric acid (35%) and 0.5-1% by weight nonionicsurfactant); U.S. Pat. No. 3,668,131 (comprising hydrogen peroxide,sulphuric acid and urea additives); Metal Finishing (February 1986), 84,(2), 67-70 (comprising sodium dichromate, sulphuric acid, hydrochloricacid, sodium diethyldithio carbonate); Trans Inst. Metal Finishing(Summer 1983), 61, (2), 46-49 (acidified hydrogen peroxide comprisinghydrogen peroxide, sulphuric acid and stabilizer); Oberfläche Surf,(August 1979) 20, (8),178-179 (comprising nitric acid and dodecylpyridinium chloride); U.S. Pat. No. 4,510,018 (comprising sulphuricacid, hydrogen peroxide, fatty acid amine and ammonium compound); U.S.Pat. No. 4,459,216 (comprising 5-100 g/l hydrogen peroxide and 100-300g/l sulphuric acid and aromatic stabilizer); JP 84-038308 (comprising0.15-0.3 moles/l hydrochloric acid; 0.2-0.4 moles/l phosphoric acid and0.02-0.1 moles/l sulphuric acid). Where the conducting material of thepads and/or through-holes comprises stainless steel, particularlypreferred briqht-etch compositions may be as described for example in WO93-08317; JP 62-238379 A2; DE 1928307; or Tr. Gos. Nauchno-Issled.Proektn. Inst. Osnovn. Khim (1974), 36, 93-97. Where the conductingmaterial is aluminium, a suitable bright-etch is as described in Met.Finishing (July 1986) 84, (7), 55-59.

[0037] Thus, any etch composition which provides a bright, cleanedsurface may be used. In the bright-etch step, contact with thebright-etch composition may be by immersion, spray or any other coatingtechnique, such as is described in any of the references above, forsufficient time and at a suitable temperature to enable a bright surfaceto form on the conducting material of the pads and/or through-holes.Generally the temperature for contact with the bright-etch compositionwill be ambient and the contact time will be from 5 seconds to 10minutes, preferably at least 30 seconds, or even at least 2 minutes, andpreferably for no greater than 5 minutes.

[0038] Generally after the etching step, there will be a post-rinse stepcomprising rinsing with deionized water and generally without drying,the bare boards then proceed directly to the plating step.Alternatively, an acid rinse step may be included, after the aqueousrinse.

[0039] The plating step is an immersion (or displacement) plating step.In an immersion plating step, the plating composition comprises metalions of a metal which is more electropositive than the conductingmaterial. The choice of metal ions in the immersion plating solution,therefore depends on the metal to be plated. Since the pads orthrough-holes generally comprise copper or nickel, suitable examples ofplating metals include bismuth, tin, palladium, silver and gold; silverand bismuth ions are particularly preferred.

[0040] A particularly preferred immersion silver plating method isdescribed in our copending British application filed on even dateherewith under the application number 9425030.5, and subsequent U.S.Ser. No. ______ (attorney docket no. A0626/7001).

[0041] As sources of plating metal ions, any water soluble metal saltmay be used, for example nitrates, acetates, sulphates, lactates orformates. Preferably silver nitrate is used.

[0042] The metal plating ions are generally in the plating compositionat a concentration of from 0.06 to 32 g/l (based on metal ions),preferably from 0.1 to 25 g/l, most preferably from 0.5 to 15 g/l.

[0043] Contact of the metal surface with the plating solution willgenerally be at temperatures of from 10 to 90° C., preferably 15 to 75°C., more preferably 20 to 60° C. For example, the temperature of contactwith the plating solution will be from 15 to 50° C., most usually from20 to 40° C.

[0044] Contact can be by any method, usually dip, spray or horizontalimmersion coating. Spray coating is preferred. Such contact may be partof a substantially continuous coating process.

[0045] The contact time of the plating solution with the metal surfaceis sufficient to form plated metal surfaces over the metal. Generallythe contact time will be from 10 seconds to 10 minutes. A contact timeof less than 10 seconds has generally being found to give insufficientcoverage with silver coating and although the contact time may be longerthan 10 minutes, no additional benefit has been found from a contacttime of longer than 10 minutes.

[0046] The preferred plating process is an immersion displacementprocess and not a true electroless plating process. In the preferredplating compositions of the present invention, metal atoms on thesurface of the metal are oxidized by the metal plating ions in thesolution, so that a layer of plated metal deposits on the pads and/orthrough-holes. The process is self-limiting because when plated metalcovers all of the surface sites of metal oxidizable by the plating metalno further reaction and therefore no further deposition occurs.

[0047] In a second aspect of the invention, there is provided forcoating a PCB comprising an insulating layer and a conducting layer,with metal pads and/or through-holes in which the pads and/orthrough-holes are provided with an anti-tarnish coating, the methodcomprising metal plating the etched pads and/or throuqh-holes by contactwith a plating composition in a metal plating step to form solderableplated metal surfaces and contacting the plated metal surfaces with asolution of a tarnish inhibitor.

[0048] In this aspect of the invention, preferably, prior to contactingthe metal surface with the plating composition in the plating step, themetal surface is cleaned. Cleaning may be using an acidic cleaningcomposition, such as any cleaning composition known in the art. Examplesare copper conditioner PC 1144 supplied by Alpha Metals Limited of theUnited Kingdom. Where there is a cleaning step using an acidic cleaningsolution, generally there will be a rinsing step prior to contacting themetal surface with the plating solution. Preferably any pre-cleaningwill include a bright-etch step. In both aspects of the invention, thetarnish inhibitor may be present in the plating solution itself so thatthe plating solution comprises the solution comprising tarnishinhibitor. Thus, in a preferred method of the invention, the platedmetal surfaces are contacted with a solution comprising a tarnishinhibitor during the plating step (i.e., contact may be during formationof the plated metal surfaces).

[0049] Alternatively, the metal surfaces are formed in the plating stepand subsequently the formed metal surfaces are contacted with a solutioncomprising a tarnish inhibitor in a further step. The solution ispreferably aqueous, being made up from deionized or otherwise purifiedwater. The composition comprising tarnish inhibitor may additionallycomprise solubilizers, for example non-aqueous solvents, surfactantsand/or pH buffers.

[0050] Contact of the composition comprising tarnish inhibitor with theplated metal surfaces will be for at least 5 seconds, preferably for atleast 20 seconds. Where the tarnish inhibitor is present in the platingsolution, the time of contact is generally determined by the duration ofthe plating step. Generally, the contact time will be from 1 to 5minutes. The temperature of contact is most usually from 10 to 90° C.,preferably 15 to 75° C., more preferably 20 to 60° C. For example thetemperature or contact with the plating solution may be from 15 to 50°C., most usually from 20 to 40° C. Contact may be any conventionalmeans, for example by dip, spray or horizontal immersion coating.

[0051] The most appropriate pH depends to some extent on the particulartarnish inhibitor used but primarily on the metal ions present in aplating bath which contains the tarnish inhibitor. Where the tarnishinhibitor is contacted in a separate step with the plated metal surface,the pH should be appropriate for the tarnish inhibitor and selected sothat it does not attack the plating. Where the solution is a silverplating composition, a convenient pH is in the range of 3 to 10. Wherethe solution is a bismuth plating composition, the pH may be 1 or lower.

[0052] The solution comprising the tarnish inhibitor may be a finalrinse solution, applied to the boards prior to drying of the boards. Theboard may undergo subsequent treatment steps after contact with thecomposition comprising tarnish inhibitor. However, generally, aftercontact with the solution, comprising a tarnish inhibitor and drying,they are at the end of the first bare board manufacturing stage, and areready for the second component-attachment stages. Optionally, forexample, there may be a deionized water rinse step, prior to drying.

[0053] The concentration of tarnish inhibitor in the solution comprisingtarnish inhibitor, will generally be from 0.0001 5 to 5% by weight, i.e.0.001 to 50 g/l. Preferably, the amount of tarnish inhibitor will befrom 0.005 to 3% by weight, and most preferably from 0.01 to 2% byweight, or even below 1% by weight.

[0054] The method of the second aspect of the invention may surprisinglyalso be used on precious metals such as gold, platinum or rutheniumwhere it will improve solderability.

[0055] In the second aspect of the invention, the metal plating step ispreferably an immersion/displacement plating or electroless platingstep. It consists of a single step using a single plating composition.Most preferably the plating step will be an immersion/displacementplating step comprising contacting the metal of the pads and/orthrough-holes with an immersion plating composition.

[0056] Where the plating is other than by the preferredimmersion/displacement process, for example, if it is by electrolessplating, the plating composition may comprise alternative plating metalions, such as nickel.

[0057] The use of tarnish inhibitor in the invention has been found toprovide metal coatings which have good tarnish resistance (resistance tohumidity and oxidation) even when stored at 40° C. and 93% RH for 96hours or at 150° C. for 2 hours. The porosity inherent in immersioncoatings is reduced by the provision of a level surface using thebright-etch step and using, so that the anti-tarnish properties areconsiderably improved, even at the high temperatures reached in reflowsoldering processes. Concern over the use of silver plating as describedfor example in DE-C-4316679 due to migration of silver ions is overcomeas it has been found that the present invention substantially preventssilver migration by providing a barrier to moisture.

[0058] In both the above aspects of the invention, an immersion platingcomposition preferably contains a complexing agent for the ions of themore electropositive metal.

[0059] In a further aspect of the present invention there is provided adisplacement metal plating process in which a relatively lesselectropositive base metal is plated with a relatively moreelectropositive coating metal by contact with an aqueous platingcomposition containing ions of the more electropositive metal, acompleting agent for such ions and a tarnish inhibitor for the moreelectropositive metal so as to form a coating of the moreelectropositive metal.

[0060] In this aspect of the invention there is also provided is a newplating composition containing ions of a metal which can be displacementplated, a completing agent for the ions, preferably present in higherthan equimolar amounts as compared to the metal ion, and containing atarnish inhibitor for the said metal, and being substantially free ofreducing agent capable of reducing the ions to the metal.

[0061] This aspect of the invention has been found to be particularlyuseful for silver or bismuth plating. Therefore preferably, the platingcomposition described contains silver or bismuth ions.

[0062] The plating composition used in this aspect of the invention maybe a immersion plating composition based on any plating composition usedin the PCB industry.

[0063] In this embodiment of the invention, preferably, prior tocontacting the metal surface with the plating composition in the platingstep, the metal surface is cleaned. Cleaning may be accomplished usingan acidic cleaning composition, such as any cleaning composition knownin the art. Examples are copper conditioner PC 1144 supplied by AlphaMetals Limited.

[0064] Where there is a cleaning step using an acidic cleaning solution,generally there will be a rinsing step prior to contacting the metalsurface with the plating solution.

[0065] Preferably any pre-cleaning will include a bright-etch step.

[0066] The plating composition may also comprise a complexing agent. Ifso, the complexing agent is preferably present in an amount of from 0.1to 250 g/l, preferably from 2 to 200 g/l and most preferably from 10 to100 g/l, especially around 50 g/l. The complexing agent may be anycomplexing agent for the plating metal ions which does not form a waterinsoluble precipitate under the aqueous and pH conditions of thecomposition. Mixtures of complexing agents may also be used. It isdesirable to use complexing agents which are bi-dentate or higherdentate ligands since the stability constants of such complexes arehigher than mono-dentate ligands.

[0067] Examples of suitable complexing agents have oxygen-containingligands, for instance amino acids and their salts, preferably having atleast 2 and up to 10 carbon atoms, polycarboxylic acids, usually aminoacetic acids, such as nitrilo-triacetic acid or, usually, alkylenepolyamine polyacetic acids including ethylene diamine tetra-acetic acid(EDTA), diethylene triamine penta-acetic acid, N-hydroxyethyl ethylenediamine triacetic acid, 1,3-diamino-2-propanol-N,N,N′,N′-tetra-aceticacid, bishydroxyphenylethylene diamine diacetic acid, diaminocyclohexane tetra-acetic acid or ethyleneglycol-bis-[(β-aminoethylether)-N,N′-tetra-acetic acid)] andN,N,N′,N′-tetrakis-(2-hydroxypropyl) ethylene diamine, citrates and/ortartrates, N,N-di-(hydroxyethyl)glycine, gluconates, lactates, citrates,tartrates, crown ethers and/or cryptands.

[0068] Particularly preferred complexing agents for silver are EDTA,DTPA and N,N,N′,N′-tetrakis-(2-hydroxypropyl) ethylene diamine. Thecomplexing agent should form a soluble complex with plating metal ionsin aqueous solution under the pH conditions of the plating solution.

[0069] A suitable complexing agent for bismuth is chloride, and it isgenerally unnecessary to use a multidentate (i.e., bi- or higherdentate) ligand complexing agent for bismuth.

[0070] The complexing agent is preferably used either in stoichiometricequivalent amounts or in a stoichiometric excess so that all the platingmetal ions may be complexed. By stoichiometric we mean equimolar.Preferably the complexing agent is present in a higher molarconcentration than the silver ions, the molar ratio preferably being (atleast 1.2):1, more preferably (at least 2.0):1, more preferably (atleast 3):1.

[0071] Suitable tarnish inhibitors for use in all aspects of the presentinvention include for example:

[0072] (a) fatty acid amines, preferably having at least 6 carbon atoms,most preferably at least 10 carbon atoms and generally no greater than30 carbon atoms, they may be primary, secondary, tertiary, diamines,amine salts, amides, ethoxylated amines, ethoxylated diamines,quaternary ammonium salts, quaternary diammonium salts, ethoxylatedquaternary ammonium salts, ethoxylated amides and amine oxides. Examplesof the primary, secondary and tertiary amine-type corrosion inhibitorsare ARMEEN™ to (™ denotes trademark). Examples of the subsequentamine-type corrosion inhibitors are respectively DUOMEEN™,ARMAC™/DUOMAC, ARMID™, ETHOMEEN™, ETHODUONEEN™, ARQUAD™, DUOQUAD™,ETHOQUAD™, ETHOMID™, AROMOX™, all supplied by Akzo Chemie.

[0073] (b) purines and substituted purines.

[0074] (c) N-acyl derivatives of sarcosine, such as the SARKOSYrange ofproducts supplied by Ciba-Geigy.

[0075] (d) organic polycarboxylic acids such as Reocor 190 supplied byCiba-Geigy.

[0076] (e) substituted imidazoline in which substituents are for examplehydroxyl C₁₋₄ alkyl amino or carbonyl-containing groups. Examplesinclude AMINE 0, produced by Ciba-Geigy, especially in combination witha N-acyl sarcosine of category (c).

[0077] (f) alkyl or alkyl benzyl imidazoles, e.g. undecyl imidazole inwhich the alkyl group has up to 22 carbon atoms, preferably no greaterthan 11 carbon atoms and in which the alkyl or benzyl groups areoptionally substituted.

[0078] (g) benzimidazoles, especially alkylaryl benzimidazoles in whichthe alkyl group has up to 22 carbon atoms, preferably no greater than 10carbon atoms and in which the alkyl or benzyl groups are optionallysubstituted, for example 2-(p-chlorobenzyl) benzimidazole which isparticularly preferred.

[0079] (h) phosphate esters such as EKCOL PS-413, supplied by Witco

[0080] (i) optionally substituted triazole derivatives such as REOMET42, supplied by Ciba-Geigy. Examples are benzo triazole, tolyl triazoleand alkyl substituted triazole derivatives having a carbon number on thealkyl group of from 1 to 22, preferably from 1 to 10.

[0081] (j) substituted tetrazoles, such as 5(3(trifluoromethyl phenyl))tetrazole, is also a preferred example.

[0082] The choice of tarnish inhibitor will depend to some extent uponthe metal of the plated metal surfaces, but this will be clear to aperson skilled in the art. For example, if the tarnish inhibitor is tobe incorporated into a gold plating bath, the tarnish inhibitor may be achloride salt, however, in contrast, using a silver plating bath,chloride salts may not be used as they will result in formation of aninsoluble silver chloride precipitate.

[0083] The tarnish inhibitor is preferably water soluble so that thesolution is an aqueous solution. However, water immiscible tarnishinhibitors may be used although it may be necessary to include asurfactant/cosolvent in the solution.

[0084] This invention has been found to provide considerable advantagesin preventing tarnishing and conferring humidity resistance on the bareboards produced to that additional protection is provided between thebare board manufacture stage and the component-attachment stage.Solderability is found to be enhanced.

[0085] A suitable pH for a silver plating composition may be from 2 to12, but is preferably from 4 to 10. Thus, the composition may be acidic,up to pH 7 Alternatively, the composition may be alkaline, and have a pHof greater than 7, or even greater than 7.5. A bismuth plating solutionusually has a low pH of 1 or less.

[0086] A buffering agent may be included in the plating composition toensure that the pH of the composition is within the desired range. Asthe buffering agent, any compatible acid or base may be included. Acompatible acid or base is an acid or base which in the amounts requiredin the composition does not result in the precipitation out of solutionof the silver ions and/or complexing agent. For example hydrogenchloride is unsuitable for a silver plating composition as it forms aninsoluble silver chloride precipitate. Suitable examples include sodiumor potassium hydroxide or a carbonate salt, or where acids are required,suitable acids may include citric acid, nitric acid or acetic acid.Borates, phthalates, acetates, phosphonates may be used but the buffershould not result in precipitation of the metal salts and preferablydoes not inhibit the plating rate. An appropriate buffer will bedependent on the desired working pH.

[0087] The plating composition may include optional ingredients such assurfactants or wetting agents to improve the coating uniformity. Wheresurfactants are included, preferably they are introduced into thecomposition in an amount such that in the plating bath, they will bepresent at a concentration of from 0.02 to 100 g/l. Preferably they willbe incorporated at a concentration of from 0.1 to 25 g/l and mostpreferably at a concentration of from 1 to 15 g/l. Any standardsurfactant or wetting agent useful in a plating bath may be usedNonionic surfactants are preferred. Particularly preferred surfactantsare alkyl phenol ethoxylates, alcohol ethoxylates and phenol ethoxylatessuch as *Synperonic NP9 (ex. ICI), *Synperonic A14 (ex. ICI) and*Ethylan HB4 (ex. Harcros), respectively (*denotes trade name).

[0088] A further optional ingredient which can be included in theplating baths of the present invention are grain refiners Grain refinersimprove the appearance of the plated metal surfaces by causing formationof smaller crystals of plated metal having a more densely parkedstructure. Suitable examples of grain refiners include lower alcoholssuch as those having from 1 to 6 carbon atoms, for example isopropanoland polyethylene glycols, for example PEG 1450 (Carbowax* UnionCarbide). Grain refiners may be incorporated in the composition inamounts from 0.02 to 200 g/l. More preferably, if a grain refiner isincluded, it will be at concentrations of from 0.05 to 100 g/l and mostpreferably from 0.1 to 10 g/l. Any nonaqueous solvent should be presentin amounts below 50% by weight of the composition, preferably below 30%by weight or even below 10% or 5% by weight of the plating composition.

[0089] Other non-active, non-interfering components may be included suchas defoamers especially for spray applications (e.g., A100 supplied byDow), dyes, etc.

[0090] The balance in the composition is water. Deionized water or otherpurified water which has had interfering ions removed, is used in theplating composition used in the process of the invention.

[0091] In order to form the plating composition for use in the processesof the present invention, preferably a solution is firstly preparedcomprising deionized water, complexing agent as defined above, and anybuffering agent, optionally with the other optional ingredients, and asalt of the more electropositive metal is added as an-aqueous solutionto the other components which have been formed into a pre-mix. It hasbeen found that this is the most advantageous way to prepare thesolution because trying to dissolve the metal salt directly into theplating composition is relatively time consuming and, where the metal issilver, tends to be more vulnerable to photo-reaction which results inprecipitation of silver ions out of solution, as a dark precipitate.

[0092] Preferably the pH of the composition to which a silver salt isadded will be from pH 3 to 10, most preferably from 4 to 8.

[0093] The components are mixed until they have substantially dissolved.The use of heat for silver dissolution is disadvantageous because again,it may tend to cause the formation of a dark silver precipitate.

[0094] After contact of the bare board with the solution comprisingtarnish inhibitor, the board is dried. Preferably, there will be nopost-rinse step between contact of the board with the solution anddrying.

[0095] Drying may be by any means, but is generally using warm air, forexample treated metal may be passed through a drying oven.

[0096] The coating obtained using the method of the present inventionproduces a surface which is considerably more uniform and even than thatobtained in the conventional HAL processes and, compared with organicprotection, the coating is more resistant to soldering operations.Furthermore, the process of this invention is less expensive and simplerthan use of the nickel/gold process.

[0097] In the subsequent component-attachment stage, the components aresoldered onto the plated pads and/or through-holes of the bare board Themetal of the pad(s) and/or through-holes (generally copper) and platingmetal, usually silver, and/or the plating metal and solder may tend tointermix. The bond formed with the components has good electricalconductivity and good bond strength.

[0098] After component attachment, finished boards having componentsattached over the plated layer of the present invention, do not sufferjoint reliability problems as do those boards formed using a nickel/goldstep.

EXAMPLE 1

[0099] A composition was prepared in which 50 g EDTA and 20.4 g of solidsodium hydroxide were mixed with sufficient water to dissolve them. Asolution comprising 1 g silver nitrate in deionized water wassubsequently added to the premixture comprising EDTA and sodiumhydroxide solution and deionized water was added to 1 liter. Copperdouble-sided circuit boards, having a variety of surface mount featureand plated through-holes of various diameter were coated with the silversolution using the following procedure.

[0100] Boards were chemically brightened in an aqueous solution of 20%v/v H₂O₂ (35%), 0.5% v/v H₂SO₄, (96%), 2.5% 1,4-butanediol for 1 minute.A tap water rinse was then employed, followed by an acid rinse in 10%H₂SO₄, for 1 minute. The boards were given a further water rinse, thenimmersed in the silver plating solution at 40° C. for 4 minutes. Afterremoval from the bath, the boards were rinsed with water and warm airdried. Copper areas of the board were coated with a bright, even silverdeposit.

[0101] Coated boards were subjected to three passes through a typical IRsilver paste reflow profile (see FIG. 1), then wave soldered usingNR300, an Alpha Metals VOC free, no clean flux. 100% filing of theplated through-holes with solder was achieved.

[0102] Further boards were stored in a humidity cabinet at 40° C./93% RHfor 24 hours before being passed through 3 IR reflow profiles. Theseboards showed a slight degree of tarnishing on the silver coating.However 100% hole filling was still achieved during subsequent wavesoldering with NR 300 flux.

EXAMPLE 2

[0103] A silver plating solution was prepared by forming a solutioncomprising 50 g EDTA, 20.4 g NaOH, 14 g Ethylan HB4 (Akros Chemicals), 3g Crodamet 02 (Croda Chemicals) in 800 mls deionized water. To thissolution was added a solution of 1 g AgNO₃ in 100 mls deionized water.The pH was adjusted to 6.8 by addition of dilute NaOH/HNO₃, then made upto 1 liter with deionized water.

[0104] Double sided bare copper boards were coated with the abovesolution using the procedure as described in Example 1. 100% filling ofplated through-holes with solder was achieved during wave soldering withNR300 flux after passage through 3 IR reflow profiles.

[0105] Boards stored at 40° C./93% RH for 24 hours prior to passagethrough 3 IR reflow profiles showed no evidence of tarnishing andsoldered well during wave soldering trials, giving 100% hole filling.

EXAMPLE 3

[0106] Double-sided bare copper boards were coated using the bathcomposition and procedure as described in Example 1. Following removalof the boards from the silver plating solution and rinsing, the boardswere immersed in a solution of 4 g Reomet 42 (Ciba-Geigy) in 1 literdeionized water (pH 7) for 1 minute at room temperature. The boards werethen rinsed in tap water and warm air dried. A bright even silvercoating was produced.

[0107] The coated boards were stored at 4° C./93% RH for 24 hours thenpassed through 3 IR paste reflow profiles. The boards showed no evidenceof tarnishing, and soldered well when wave soldered using KR 300 flux.

EXAMPLE 4

[0108] Coupons of copper strip (5 cm×1 cm) were coated with the silvercoating as described in Example 2. In addition, further samples werecoated with immersion tin, 63/37 Sn/Pb and two competitors solderabilitypreservative coatings based on substituted benzimidazole chemistry. Thefollowing coating procedures were applied for the various samples:

[0109] Immersion Tin Coating

[0110] Coupons were etched in an aqueous solution of Na₂S₂O₈ (5%), H₂SO₄(5%) for 2 minutes, rinsed with tap water, then rinsed with 10% H₂SO₄,for 1 minute and then rinsed with deionized water The coupons were thenimmersed in an immersion tin plating solution comprising 0.33 g/lSn(BF₄)₂ 150 g/l thiourea, 20 g/l fluoroboric acid and 5 g/l SynparonicNP9 (ex. ICI) in deionized water, for 1 minute at room temperature. Thecoupons were then rinsed with deionized water and warm air dried.

[0111] Sn/Pb Coating

[0112] Coupons were etched in an aqueous solution comprising Na₂S₂O₈,(5%) and H₂SO₄ (5%), rinsed with tap water then with 10% H₂SO₄ and thenwith deionized water. The coupons were warm air dried. Alpha NR 300 fluxwas then applied to each coupon. The coupons were then coated 63/37Sn/Pb by immersion in molten solder at 250° C. for 3 seconds.

[0113] Azole 1 and Azole 2

[0114] Coupons were etched and rinsed as for the immersion tin samples.Coupons were then immersed in the solution containing the azole at 40°C. for 90 seconds. After removal from the azole containing solution, thecoupons were rinsed with deionized water, and warm air dried.

[0115] The coupons were subjected to a variety of differentpretreatments. A. No pre-treatment. B. Passage through 3 solder pastereflow profiles. C. Storage at 40° C./93% RH for 96 hours. D. Storage at40° C./94% RH for 96 hours, then 3 solder paste reflow profiles. E.Storage at 150° C. for 2 hours.

[0116] Samples were then soldered using a meniscograph with NR300 flux.

[0117] The meniscograph test method monitors the solderability bymeasuring the net force acting between specimen and solder. The coatingsare assessed by the length of time to reach zero wetting force, and thesize of the equilibrium wetting force. To achieve good results in wavesoldering a short wetting time and high equilibrium wetting force arepreferred.

[0118] The table below shows the wetting times in seconds and wettingforces after 2 seconds immersion in mN/mm for various copper coatedsamples.

[0119] As can be seen from above, the silver coatings prepared accordingto this invention have shorter wetting times and higher wetting forcesthan the Sn and benzimidazole alternative and retain these propertiesmore readily after humidity and heat treatment. TABLE 1 Wet WettingForce Coating Pre-Treatment Time/Sec at 2 Seconds Example 2 A 0.7 0.429Example 2 B 0.8 0.444 Example 2 C 0.7 0.429 Example 2 D 0.7 0.441Example 2 E 0.8 0.438 Tin A 0.9 0.488 Tin B >5 −0.028 Tin C >5 0.008 TinD >5 −0.148 Azole 1 A 0.8 0.439 Azole 1 B 0.9 0.412 Azole 1 C 0.9 0.443Azole 1 D 0.9 0.426 Azole 1 E 1.0 0.421 Azole 2 A 0.9 0.449 Azole 2 B1.0 0.417 Azole 2 C 0.9 0.466 Azole 2 D 1.1 0.310 Azole 2 E 1.2 0.296Sn/Pb A 0.8 0.475 Sn/Pb B 0.8 0.501 Sn/Pb C 0.8 0.492 Sn/Pb D 0.8 0.474Sn/Pb E 0.8 0.492

EXAMPLE 5

[0120] A displacement bismuth plating composition was preparedcomprising 3.9 g bismuth oxide, 183.lg hydrogen chloride (as 37%solution), 490.Sg glycolic acid (70% solution), 265.4 g (50% sodiumhydroxide solution), 0.077 g potassium iodide, 0.003 g Synperonic NP9(ex. ICI) and 4 g 2p-chlorobenzyl benzimidazole, were added to deionizedwater to make 1 liter of product solution Bare boards having copper padsand copper through-holes were chemically brightened as described inExample 1, then immersed in the plating baths for 2 minutes at 70° C. Acoating of bismuth was formed on the surface of the copper having athickness of 0.05 μm. Subsequent solderability and tarnish resistancetests carried out on the plated bare boards showed good results forsolderability and tarnish resistance.

EXAMPLE 6

[0121] Double-sided bare copper boards were bright etched in an aqueoussolution of 50% v/v HNO₃, 10% H₂SO₄, 10% H₃PO₄, 1% HCl for 1 minute atroom temperature. Boards were then rinsed in tap water followed by 10%H₂SO₄ for 1 minute. After a further water rinse, boards were immersed insilver plating bath described in Example 2 for 4 minutes at 45° C.Boards were then water rinsed and warm air dried.

[0122] The coated boards were stored at 40° C./93% RH for 24 hours thenpassed through 3 IR paste reflow profiles. The boards showed no evidenceof tarnishing when wave soldered using NR 300 flux.

EXAMPLE 7

[0123] A silver plating bath was prepared by forming a solutioncomprising 64.8 g diethylene triamine penta-acetic acid, 23.0 g NaOH, 24g surfactant Ethylan HB4 (Akros Chemicals), 2.5 g Crodamet 02 anethoxylated 3° amine compound (Croda Chemicals) in 800 mls deionizedwater. To this solution was added a solution of 1 g silver nitrate in100 mls deionized water. The pH of this solution was adjusted to 6.9 byaddition of dilute NaOH solution or nitric acid. The volume was thenmade up to 1 liter using deionized water.

[0124] Double-sided bare copper boards were coated using the abovesolution using the procedure as described in Example 1. 100% filling ofthe plated through-holes was achieved during wave-soldering of thecoated boards using Alpha Metals MR300 flux after passage through 3 IRreflow profiles showed no evidence of tarnishing, and soldered wellduring wave-soldering trials giving 100% hole-fill.

EXAMPLE 8

[0125] An immersion silver plating solution was prepared comprising 98.2g deionized water, 1 g of nitric acid, 0.1 g of silver nitrate, 0.3 g ofChemeen C2 (antitarnish) and 0.4 g Mazawet DF (solubilizer). The pH wasadjusted to 6 using a 50% solution of ethylene diamine. The bathproduced an adherent silver deposit on copper coupons which showed goodsolderability and humidity resistance.

EXAMPLE 9

[0126] A bismuth plating solution was prepared containing bismuthtrioxide 2.1% weight, hydrochloric acid (22° Be) 46.73% weight,glycollic acid (70%) 49.5% weight, potassium chloride 0.07% weight,polyethylene glycol 600 0.1% weight, Chemax Chemeen C2 0.2% weight,distilled water 1.2% weight and tartaric acid 0.1% weight. A furthersolution was prepared from which the Chemeen C2 was omitted. Samples ofcopper clad printed circuit material were plated in each of thesolutions. These plated samples were then placed in a humidity chamberfor 16 hours at 60° C. and 95% relative humidity.

[0127] After this exposure the samples were examined, and those preparedin the solution without the Chemeen C2 were heavily tarnished. Thesamples prepared in the solution containing the tarnish inhibitor had agood appearance with minimal oxidation, and when tested showed goodsolderability.

[0128] Other embodiments of the invention are to be considered withinthe scope of the appended claims.

What is claimed is:
 1. A plating solution comprising: a solvent;metallic ions dissolved in the solvent; and a tarnish inhibitor in thesolvent, the tarnish inhibitor characterized in that it does not causethe dissolved metallic ions to precipitate in the solution.
 2. Theplating solution of claim 1, wherein the metallic ions are selected fromthe group consisting of nickel, silver, tin, lead, palladium, cobalt,gold, platinum, bismuth and combinations thereof.
 3. The platingsolution of claim 2, wherein the metallic ions include silver ions. 4.The plating solution of claim 1, wherein the solvent comprises water. 5.The plating solution of claim 1, wherein the tarnish inhibitor ispresent at a concentration in the range from 0.001 to 50 g/l.
 6. Theplating solution of claim 5, wherein the tarnish inhibitor is present ata concentration in the range from 0.05 to 30 g/l.
 7. The platingsolution of claim 6, wherein the tarnish inhibitor is present at aconcentration in the range from 0.1 to 20 g/l.
 8. The plating solutionof claim 7, wherein the tarnish inhibitor is present at a concentrationof less than 10 g/l.
 9. The plating solution of claim 1, furthercomprising a complexing agent for the dissolved metallic ions.
 10. Theplating solution of claim 9, wherein the complexing agent includes amultidentate ligand.
 11. The plating solution of claim 10, wherein thecomplexing agent includes an oxygen-containing ligand.
 12. The platingsolution of claim 1, wherein the complexing agent is present at aconcentration that is the stoichiometric equivalent or greater than theconcentration of the dissolved metallic ions.
 13. The plating solutionof claim 1, wherein the complexing agent is present at a concentrationof 0.1 to 250 g/l.
 14. The plating solution of claim 13, wherein thecomplexing agent is present at a concentration of 2 to 200 g/l.
 15. Theplating solution of claim 14, wherein the complexing agent is present ata concentration of 10 to 100 g/l.
 16. The plating solution of claim 15,wherein the complexing agent is present at a concentration of about 50g/l.
 17. The plating solution of claim 1, wherein the plating solutionhas a pH in the range from 2 to
 12. 18. The plating solution of claim17, wherein the plating solution has a pH in the range from 4 to
 10. 19.The plating solution of claim 18, wherein the plating solution has a pHof about
 7. 20. A silver-plating solution comprising: water; a source ofsilver ions dissolved in the water; and a tarnish inhibitor in the waterat a concentration from 0.0001 to 5% by weight of the silver-platingsolution, the tarnish inhibitor characterized in that it does not causethe dissolved silver ions to precipitate in the solution.
 21. Thesilver-plating solution of claim 20, wherein the tarnish inhibitor is ata concentration in the range from 0.005 to 3% by weight.
 22. Thesilver-plating solution of claim 20, wherein the tarnish inhibitor is ata concentration in the range from 0.01 to 2% by weight.
 23. Thesilver-plating solution of claim 20, wherein the plating solution has apH in the range from 4 to
 10. 24. The silver-plating solution of claim23, further comprising a complexing agent for the silver ions.
 25. Thesilver-plating solution of claim 24, wherein the complexing agent is amultidentate ligand.
 26. The silver-plating solution of claim 25,wherein the complexing agent is present at a concentration that is thestoichiometric equivalent or greater than the concentration of thedissolved metallic ions.
 27. The silver-plating solution of claim 26,wherein the complexing agent is present at a concentration of 0.1 to 250g/l.
 28. An aqueous plating composition suitable for forming animmersion plating of a first metal having a first electropositivity on asubstrate formed of a second metal having a second electropositivity,the first electropositivity being greater than the secondelectropositivity, the composition comprising: ions of the first metal;a complexing agent for the ions of the first metal; and a tarnishinhibitor for the first metal, the composition being substantially freeof any reducing agent for the ions of the first metal.
 29. A compositionaccording to claim 28, wherein the tarnish inhibitor composes from 0.001to 5% by weight of the composition.
 30. A composition according to claim28, wherein the ions of the first metal are selected from the groupconsisting of nickel ions, silver ions, tin ions, lead ions, palladiumions, cobalt ions, gold ions, platinum ions, bismuth ions and mixturesthereof.
 31. A composition according to claim 30, wherein the ions ofthe first metal are silver ions.